1. Field of the Invention
The present invention relates to the purification of impure white phosphorus values.
This invention also relates to apparatus for carrying out such purification of impure white phosphorus.
2. Description of the Prior Art
White phosphorus is essentially produced industrially by reduction of phosphates by carbon in the presence of silica in an electric furnace.
The white phosphorus produced by this electrothermal process contains a small amount of impurities, which may be dissolved or remain in the solid state and are of organic nature (hydrocarbons and phenol compounds) or inorganic nature (iron, arsenic, and the like).
These impurities are not generally detrimental for a large number of uses of the white phosphorus. However, certain preparative processes of phosphorus-containing derivatives, in particular the preparation of P.sub.4 S.sub.10, require using a white phosphorus starting material of high purity and virtually devoid of organic compounds.
Indeed, the presence of such compounds causes the formation of colored products which are difficult to separate off.
Numerous patents (e.g., French Patent No. 2,057,536 and German Patent No. 2,035,432) disclose that white phosphorus originating from the electrothermal process may be purified by treating it with H.sub.2 SO.sub.4 under variable conditions of concentration and temperature.
These procedures, however, present certain disadvantages.
On the one hand, peroxides may form, with attendant risk of violent and uncontrollable reactions.
On the other, despite numerous washings, a not insignificant amount of H.sub.2 SO.sub.4 (up to 50 ppm) remains which occasions risks of corrosion.
Moreover, although this treatment has a certain efficiency with regard to the removal of organophosphorus materials, it only slightly reduces the content of suspended material in the purified phosphorus.
Finally, these processes produce large amounts of aqueous effluents--more than 2 tons of 5% sulfuric acid polluted by organic matter per ton of treated phosphorus--these amounts militating against the economy of the process and being environmentally damaging.
U.S. Pat. No. 4,664,896 describes that liquid white phosphorus may be purified by treatment with active charcoal.
In this process, the filter cake essentially containing spent active charcoal, phosphorus, impurities removed from the purified phosphorus and the filter agent or material is thinned with liquid white phosphorus such as to provide a transportable slurry.
This slurry is recycled to the electric furnace for the production of phosphorus.
This procedure presents certain disadvantages, such as, for example, the recycling of the phosphorus-containing slurry to the electric furnace. Such slurry inevitably contains water in a not insignificant amount, introduced, for example, by the filter agent or the spent active charcoal. This water, in the presence of phosphorus and carbon, under the elevated temperature conditions of the electric furnace, is liable to result in the formation of hydrogen, with attendant risk of explosion, and of phosphine PH.sub.3. This toxic compound is of a type which pollutes gaseous effluents.
Furthermore, its formation gives rise to a useless and not insignificant consumption of phosphorus.
Moreover, the use of liquid phosphorus to produce a transportable slurry renders the process expensive and, in particular, requires a supplementary investment for completely safe transport of the liquid white phosphorus used in this manner.